Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.



Thomas, Sakrete patching materials will bond to itself, however you do not want to exceed the 24hr time period in which all layers should be finished. Exceeding the 24hr window will more than likely compromise the cure and bond.
- Chris Technical Services
Thursday, September 3, 2015 at 9:01 AM

Mike, as you pointed out the ramps are "very" old. there are a few things to consider when you add new concrete to old/existing concrete. If the existing concrete is not solid and sound you may be better off starting fresh. The ramp will likely need to be extended to create a suitable slope for your vehicle. If you choose to keep the existing ramps be sure to properly prep the area by removing any dirt, debris, loose material, oil, paint, stain, etc. Sakrete Concrete Bonder and Fortifier can be used as a primer/bonding agent by brushing a light coat on the clean surface first. Use Sakrete sand mix for 2" down to 1/2" and Sakrete High Strength Concrete Mix for 2" or greater.
- Chris Technical Services
Friday, August 28, 2015 at 1:04 PM

i was wondering if sakrete could be used to bond to itself i am sealing a small pond but the project will take several days will the sakrete bond to its self?
- thomas
Friday, August 28, 2015 at 12:22 PM

I have some *very* old concrete ramps to get into an equipment shed. Unfortunately they're too steep to use for my low clearance car, so I want to make them less steep. What would be the best method to do this without removing the old ramps first?
- Mike
Friday, August 28, 2015 at 7:14 AM

Dave, unfortunately we do not have a product in our line that we suggest for this type of repair. You may want to check with your local planning dept. for suggested materials or alternatives to correct this issue.
- Chris Technical Services
Tuesday, August 25, 2015 at 3:47 PM

What product(s) would be best to repair the hole in my septic tank where the drain pipe (PVC) exits to the drainfield. The concrete has worn away over the years leaving a hole that is 1/2" larger around the pipe. On the outside of the tank at this hole, there is a depression approx. 1/2" deep also.
- Dave
Monday, August 24, 2015 at 9:31 PM

Michael, Sakrete Sand Mix can be used for your project/application. As far as imprinting the concrete using molds we advise doing a trial/test area first so that timing and finish is verified before you begin.
- Chris Technical Services
Monday, August 24, 2015 at 2:58 PM

Stan, depending on temperature and humidity Sakrete Sand Mix allows foot traffic after minimum 24hrs and vehicle traffic after 3-5 days.
- Chris Technical Services
Monday, August 24, 2015 at 2:54 PM

I'm dong a patio resurface and want to use cement molds to imprint a pattern on the fresh surface. The resurfaced layer will be about an 1" to and 1 1/2" thick. Would the sand mix be an appropriate material for this repair or can you suggest another product?
- michael
Sunday, August 23, 2015 at 8:43 PM

How long would I have to wait before walking on the Sakrete Sand Mix? Thank You.
- Stan
Friday, August 21, 2015 at 1:13 AM



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